Cladding-pumped fiber devices, such as lasers and amplifiers, are important in a wide variety of optical applications. In optical communications, cladding-pumped lasers they are used to pump high power Er/Yb amplifiers, to pump remotely located Er amplifiers in repeaterless communications systems and to pump Raman lasers and amplifiers. In addition cladding pumped fiber devices have promising applications as light sources for printers and in medical optics as well as materials processing.
A typical cladding-pumped fiber device comprises a single-mode core and a plurality of cladding layers. The inner cladding surrounding the core is typically a silica cladding of large cross-sectional area (as compared to the core) and high numerical aperature. It is usually non-circular (rectangular or star-shaped) to ensure that the modes of the inner cladding will have good overlap with the core. The outer cladding is commonly composed of a low refractive index polymer. The index of the core is greater than that of the inner cladding which, in turn, is greater than the outer cladding.
A major advantage of the cladding pumped fiber is that it can convert light from low-brightness sources into light of high brightness in a single mode fiber. Light from low brightness sources such as diode arrays can be coupled into the inner cladding due to its large cross-sectional area and high numerical aperature. In a cladding pumped laser or amplifier the core is doped with a rare earth such as Er. The light in the cladding interacts with the core and is absorbed by the rare-earth dopant. If an optical signal is passed through the pumped core, it will be amplified. Or if optical feedback is provided (as by providing a Bragg grating optical cavity), the cladding-pumped fiber will act as a laser oscillator at the feedback wavelength.
A difficulty preventing full exploitation of the potential of cladding-pumped fiber devices is the problem of coupling a sufficient number of low brightness sources into the inner cladding efficiently. A common approach is to couple the light from broad-stripe semiconductor lasers into multimode fibers, to bundle the fibers and then to use bulk optics to couple the light from the bundle into the cladding-pumped fiber. See, for example, U.S. Pat. No. 5,268,978. The difficulty with this approach, however, is that it requires a number of fine interfaces with associated problems of matching and alignment, as well as two sets of fiber optics. An astigmatic lens is typically disposed between the semiconductor lasers and the bundling fibers and between the bundling fibers and the fiber laser. Polishing, antireflection coatings and maintenance of precise alignments are also required. Accordingly there is a need for a new robust and compact arrangement for efficiently coupling the output of low-brightness sources into cladding-pumped fibers.
Another difficulty preventing full exploitation of the potential of cladding-pumped fiber lasers and amplifiers is the problem of coupling multimode pump light into the inner cladding while simultaneously coupling single-mode light out of or into the single-mode core. The ability to perform this function would allow one to construct bidirectionally pumped cladding-pumped fiber lasers. Pump light could be injected into the inner cladding of both ends of the cladding-pumped fiber while the single-mode fiber laser output could be extracted from the core. Additionally, simultaneous single-mode and multimode coupling to a cladding-pumped fiber would allow one to construct cladding-pumped fiber amplifiers much more efficient than conventional single-mode fiber amplifiers pumped by cladding-pumped fiber lasers. Clearly there is a need for an efficient means of simultaneously coupling multimode pump light into the inner cladding of a cladding-pumped fiber while coupling single-mode light into or out of the core.